To determine the density of green on a patch of land, researchers
must observe the distinct colors (wavelengths) of visible and
near-infrared sunlight reflected by the plants. As can be seen through
a prism, many different wavelengths make up the spectrum of sunlight.
When sunlight strikes objects, certain wavelengths of this spectrum are
absorbed and other wavelengths are reflected. The pigment in plant
leaves, chlorophyll, strongly absorbs visible light (from 0.4 to 0.7 µm)
for use in photosynthesis. The cell structure of the leaves, on the
other hand, strongly reflects near-infrared light (from 0.7 to 1.1 µm).
The more leaves a plant has, the more these wavelengths of light are
affected, respectively.

Vegetation appears very different
at visible and near-infrared wavelengths. In visible light (top), vegetated areas are very dark,
almost black, while desert regions (like the Sahara) are light. At near-infrared wavelengths,
the vegetation is brighter and deserts are about the same. By comparing visible and infrared light,
scientists measure the relative amount of vegetation. (The variation in shade is more apparent in
the detail of the U.S. West Coast).

The NOAA AHVRR instrument has five detectors, two of which are
sensitive to the wavelengths of light ranging from 0.550.70 and
0.731.0 micrometers. With AHVRRs detectors, researchers
can measure the intensity of light coming off the Earth in visible and
near-infrared wavelengths and quantify the photosynthetic capacity of
the vegetation in a given pixel (an AVHRR pixel is 1 square km) of land
surface. In general, if there is much more reflected radiation in
near-infrared wavelengths than in visible wavelengths, then the
vegetation in that pixel is likely to be dense and may contain some type
of forest. If there is very little difference in the intensity of
visible and near-infrared wavelengths reflected, then the vegetation is
probably sparse and may consist of grassland, tundra, or desert.

NDVI is calculated
from the visible and near-infrared light reflected by vegetation. Healthy
vegetation (left) absorbs most of the visible light that hits it, and reflects a
large portion of the near-infrared light. Unhealthy or sparse vegetation (right)
reflects more visible light and less near-infrared light. The numbers on the figure
above are representative of actual values, but real vegetation is much more varied.
(Illustration by Robert Simmon).

Nearly all satellite Vegetation Indices employ this difference
formula to quantify the density of plant growth on the Earth 
near-infrared radiation minus visible radiation divided by near-infrared
radiation plus visible radiation. The result of this formula is called
the Normalized Difference Vegetation Index (NDVI). Written
mathematically, the formula is:

NDVI = (NIR  VIS)/(NIR + VIS)

Calculations of NDVI for a given pixel always result in a number that
ranges from minus one (-1) to plus one (+1); however, no green leaves
gives a value close to zero. A zero means no vegetation and close to +1
(0.8 - 0.9) indicates the highest possible density of green leaves.